1. Field of the Invention
The present invention relates to a process for the removal of carbonaceous deposits from walls of a vapor passageway. More particularly, it is concerned with the removal and control of coke deposition from the interior surfaces of a cyclone separator outlet nozzle leading to a scrubbing unit such as employed in a fluid coking process, by the application of thermal shock treatment and controlled oxidation.
2. Description of the Prior Art
Fluid coking is a well-known process, see for example U.S. Pat. No. 2,881,130. Briefly described, an oil feed is injected into a fluidized bed of solid contact particles which are usually coke particles maintained at a temperature ranging between about 850.degree. to about 1,200.degree. F for fuel production and up to 2,000.degree. F for chemicals production. Although coke particles are preferred, the contact solids having a size range of 0 to 1,000 microns, such as sand, silica, mullite, etc. may be alternatively utilized.
Upon contact with the hot solids, the oil is pyrolitically converted to lighter vaporous material and a solid carbonaceous material (coke) which deposits on the contact solids. Vapors are withdrawn overhead and sent to a separation zone, generally one or more cyclone separators wherein entrained solids are removed. Vapors thereafter are passed to a scrubbing or fractionation zone for the recovery of the desired components.
Heat for the process is supplied conventionally by circulating contact particles to a burner zone wherein oxidation of carbonaceous deposits serves to heat them to supply the requisite thermal energy for the conversion step upon their recirculation to the coking reaction bed.
Typical feeds utilized in fluid coking have initial boiling points ranging from about 700.degree. F, and an API gravity of about 0.degree. to 20.degree. and a Conradson carbon residue content of about 5 to about 50 weight percent.
Considerable difficulty has been encountered in the operation of cyclone separators employed to remove entrained solids. The vapors, upflowing from the reaction bed, contain heavy hydrocarbon fractions which have a tendency to condense on exposed surfaces, thereby depositing carbonaceous residues. The vapor outlet conduit of the cyclone is of considerably narrower dimensions than the reaction vessel, separation chamber or other passageway to which the coker vapors flow. Hence, there is a much greater tendency for carbonaceous deposits to build up to appreciable levels and interfere with or completely stop the normal flow of vapors. Since the cyclone outlet conduit normally serves as the vapor inlet to a quench zone, the entrance to which is approximately 200.degree. to 300.degree. F colder than the temperature of the reaction bed, there is a decided influence to promote condensation of heavier hydrocarbons within the passageway and the consequent deposition of coke material. The hydrocarbon conversion unit may have to be shut down to remove these coke deposits.
To overcome this difficulty, heretofore, various methods have been proposed to remove coke or minimize coke deposits in cyclone outlets. These methods include onstream methods which do not interfere with the flow of material and the conversion process as well as methods which require shut down of the unit.
To minimize or prevent condensation of vapors in the cyclone outlet, it has been proposed to increase the temperature of the cyclone outlet by various heat exchange means to minimize the condensation of vapors. Generally, such heat exchange processes have not eliminated the need for periodic coke removal. A conventionally utilized method for coke removal comprises inserting a water lance through the scrubber vessel shell for each cyclone when decoking is required. During insertion and retraction of the lance, a packing gland is required and the vessel shell is venerable to thermal shock by misdirected water. The shocking of the scrubber shell by water can cause shell cracking.
It is known to use a mixture of steam and an oxygen-containing gas to remove carbon from pipes or retorts (see, for example, U.S. Pat. No. 49,989 and U.S. Pat. No. 1,470,359).
It is also known to spall coke from vessel walls by hydraulic means, such as streams of high pressure water (see, for example, U.S. Pat. No. 3,745,110).
The present invention sets forth a process and apparatus for decoking and limiting or preventing the concentration of residue in a cyclone vapor outlet without substantially interfering with the normal flow of vapor materials or the operation of the conversion reaction.